[HDFS-8693] refreshNamenodes does not support adding a new standby to a running DN

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 2.6.0
• Fix Version/s: 3.1.0, 2.10.0, 2.9.1, 3.0.1, 2.8.4
• Component/s: datanode, ha
• Hadoop Flags: Reviewed
• Assignee: Ajith S
• Reporter: Jian Fang
• Created: 29/Jun/15 17:32
• Updated: 15/Feb/18 07:12
• Resolved: 12/Feb/18 17:09

Description

I tried to run the following command on a Hadoop 2.6.0 cluster with HA support

$ hdfs dfsadmin -refreshNamenodes datanode-host:port

to refresh name nodes on data nodes after I replaced one name node with a new one so that I don't need to restart the data nodes. However, I got the following error:

refreshNamenodes: HA does not currently support adding a new standby to a running DN. Please do a rolling restart of DNs to reconfigure the list of NNs.

I checked the 2.6.0 code and the error was thrown by the following code snippet, which led me to this JIRA.

void refreshNNList(ArrayList<InetSocketAddress> addrs) throws IOException

{ Set<InetSocketAddress> oldAddrs = Sets.newHashSet(); for (BPServiceActor actor : bpServices) { oldAddrs.add(actor.getNNSocketAddress()); }

Set<InetSocketAddress> newAddrs = Sets.newHashSet(addrs);
if (!Sets.symmetricDifference(oldAddrs, newAddrs).isEmpty())

{ // Keep things simple for now -- we can implement this at a later date. throw new IOException( "HA does not currently support adding a new standby to a running DN. " + "Please do a rolling restart of DNs to reconfigure the list of NNs."); }

}

Looks like this the refreshNameNodes command is an uncompleted feature.

Unfortunately, the new name node on a replacement is critical for auto provisioning a hadoop cluster with HDFS HA support. Without this support, the HA feature could not really be used. I also observed that the new standby name node on the replacement instance could stuck in safe mode because no data nodes check in with it. Even with a rolling restart, it may take quite some time to restart all data nodes if we have a big cluster, for example, with 4000 data nodes, let alone restarting DN is way too intrusive and it is not a preferable operation in production. It also increases the chance for a double failure because the standby name node is not really ready for a failover in the case that the current active name node fails.

Issue Links

• is related to: HDFS-1623 High Availability Framework for HDFS NN